1. Field
Exemplary embodiments of the present invention relate to a memory device.
2. Description of the Related Art
A memory includes a plurality of memory cells. Each memory cell of a memory includes a transistor serving as a switch and a capacitor storing a charge, which represents data. Data is distinguished between a logic high level (logic value ‘1’) and a logic low level (logic value ‘0’) depending on whether there is a charge in a capacitor of a memory cell, in other words, whether the terminal voltage level of the capacitor is high or low.
Data is stored by way of charges accumulated in the capacitor and there is no loss of the stored data or the accumulated charges in theory. However, because of current leakage in the PN junction of a MOS transistor, the initial charge accumulated in the capacitor may disappear, and consequently the data stored in the memory cell may be lost. To prevent data loss, the data stored in the memory cell is read and the charge has to be recharged based on the read data periodically before the stored data is lost, which is called a refresh operation. The refresh operation makes the memory retain the stored data without data loss.
FIG. 1 is a circuit diagram illustrating a portion of a cell array included in a memory device to describe a word line disturbance phenomenon. In FIG. 1, BL represents a bit line.
Referring to FIG. 1, three word lines WLK−1, WLK, and WLK+1 are arranged in parallel in the cell array. The word line WLK with notation “HIGH_ACT” is an activated word line. The word lines WLK−1 and WLK+1 are disposed adjacent to the word line WLK. Also, memory cells CELL_K−1, CELL_K, and CELL_K+1 are coupled to the word lines WLK−1, WLK, and WLK+1, respectively. The memory cells CELL_K−1, CELL_K and CELL_K+1 include cell transistors TR_K−1, TR_K, and TR_K+1 and cell capacitors CAP_K−1, CAP_K, and CAP_K+1, respectively.
When the word line WLK is activated, voltages of the word lines WLK−1 and WLK+1 fluctuate due to a coupling effect between the word line WLK and the word lines WLK−1 and WLK+1, thereby affecting the amount of charge stored in the cell capacitors CAP_K−1 and CAP_K+1 of the word lines WLK−1 and WLK+1, which is referred to as word line disturbance. In other words, the probability of data loss of the memory cells CELL_K−1 and CELL_K+1 corresponding to the neighboring word lines WLK−1 and WLK+1 of the active word line WLK increases, which may become more severe as the active word line WLK is activated for a longer period.